Remove thin server dependency (development)

I believe we still include thin server as development dependency in our Gemfile for historical reasons (we used to ship unicorn for production, which was not the best server to use when doing development).

Puma does a much better job serving as both development and production server, so there is no obvious reason to keep shipping thin other than maybe someone still relies on it?

So I'm curious if anyone has any strong reason against removing it.

---

To be clear, this does not impact production in any form, as thin is a dependency installed only when using GitLab in development mode (like when using GDK, for example).

What triggered this desire to remove it right now is that it seems there are some compatibility issues with CLang 12, that you can read here: gitlab-development-kit#1034 (comment 399740572) (there seesm to have fixes for those already, but instead of waiting for a release and upgrading that dependency we could simply remove it)